This invention relates to a blue phosphor for use in thin-film electroluminescent (TFEL) devices such as electronically-controlled matrix display panels, the blue phosphor comprising a strontium sulphide (SrS) host material doped with cerium fluoride (CeF.sub.3) in a concentration sufficient to provide a significant source of visible light photons.
There has been a need for the development of an efficient blue light phosphor for use in TFEL devices. While efficient phosphors for the other primary colors have been developed for solid-state TFEL devices, the brightest TFEL blue phosphor heretofore known is a zinc sulphide host material doped with thulium fluoride (ZnS:TmF.sub.3) which has a brightness of 0.33 foot lamberts (fL) at 1 kHz drive excitation. In addition to the fact that the eye is relatively insensitive to the blue part of the visible light spectrum, an additional factor which has rendered efficient solid-state blue emitters so difficult to develop is the large energy transfer required for their excitation because so much of the energy is dissipated in other competing lower energy channels such as optical phonons, donor-acceptor pairs, or lower energy atomic transitions before the blue center is excited. Without an efficient blue phosphor, full-color TFEL display panels, usable in room-light environments, have not been practical.
The use of SrS and other alkaline earth sulphides such as CaS and BaS as host materials for luminescent phosphors, such as those used in powder electroluminescent devices, is disclosed by A. Vecht et al. in "DCEL Dot Matrix Displays in a Range of Colors," Digest of the 1980 Society for Information Display International Symposium, pp. 110-111, and also in Highton and Vecht's U.S. Pat. No. 4,365,184. In addition, the following references describe CRT blue or other primary color powder phosphors using strontium sulphide or other alkaline earth sulphide phosphors:
W. Lehman, "Alkaline Earth Sulfide Phosphors Activated by Copper, Silver, and Gold," J. Electrochem Soc. 117, p. 1389 (1970);
W. Lehman, "On the Optimum Efficiency of Cathodoluminescence of Inorganic Phosphors," J. Electrochem Soc. 118, p. 1164 (1974);
W. Lehman and F. M. Ryan, "Cathodoluminescence of CaS:Ce and CaS:Eu Phosphors," J. Electrochem Soc. 118, p. 477 (1971).
The work by these prior investigators established that SrS, together with CaS, are efficient phosphor hosts in powder form for use in DC-excited and cathodoluminescent applications, but, because of the inherent differences with solid-state TFEL devices, as regards their respective excitation mechanism and structure, no direct correlation could be made or inferred that SrS would likewise be an efficient host material for an AC-excited blue phosphor TFEL device.